1. Field of the Invention
This invention relates to color effect pigments, to processes by which these pigments are made and, more particularly, to the use thereof to provide a goniochromatic finish.
2. Prior Art
Goniochromaticity is the effect of perceived color varying as the angle of illumination or observation varies. Goniochromatic pigments are used, for example, in automotive coatings, decorative coatings, plastic pigmentation, printing inks (security inks in particular), textiles, and cosmetics. Their optical effect results from the directional reflection of light from predominantly sheet-like particles that conventionally are metallic or that have a structured refractive index contrast, the length scale of which is comparable to the wavelength of light. According to the nature of the pigment particles, the pigments are known as metallic effect pigments (for example, aluminum, zinc, copper or alloys thereof) or interference pigments (for example, based on titanium dioxide-coated mica, such as muscovite, phlogopite, and biotite).
As a result of the incident light being reflected directionally by the predominantly sheet-like particles, color effect pigments that are oriented, for example, in a coating, exhibit goniochromaticity; that is, their perceived color (lightness and/or hue and/or chroma) varies with the angle of illumination or observation.
Interference pigments may comprise a single plate-like layer, or a multilayer structure. The color perceived is affected by, for example, interference in the thin layer or layers, and optionally also by absorption by a chromophore (an organic moiety or inorganic complex that absorbs wavelengths of light in the visible and/or UV ranges) or a color center. A color center is an electron hole pair that results from a lattice defect in a crystalline solid-state material and which absorbs wavelengths in the visible and/or UV ranges. Interference, with or without absorption, results in a multiplicity of hue variations that are dependent on the thickness of the thin layer or layers and the effective refractive index of the layer or layers.
Color effect pigments that rely upon interference phenomena generated by the presence of one or more thin layers to develop high chroma (i.e., purity of color) generally use one to five thin layers of materials that have high refractive index contrast. Examples of this type are generally known and include, but are not limited to, metal oxide-coated silicatic (including mica) and metallic pigments. The density of such metal-containing materials is typically 2 to 4 times greater than the density of their surrounding coating composition (e.g. in a paint). As a result, these metal-containing materials tend to settle out which may result in a non-uniform color effect of the coating composition.
One approach that avoids the problems associated with metal-containing materials has been in the use of organic liquid crystals, such as disclosed in U.S. Pat. No. 5,824,733. However, liquid crystals are not as physically durable as metal-containing color effect materials in a coating composition and their highly aromatic composition is subject to photodegradation with concomitant change or loss of their color effect which is highly undesirable.
While pigment particles of multilayer metal-containing materials have been successfully used for their angle-dependent optical properties, multilayered organic materials have been limited to use in film form. U.S. Pat. No. 5,122,905 describes a multilayered organic film for use as a reflective sheet or body. Similarly, U.S. Pat. No. 5,783,120 discloses an optical film of polymeric particles dispersed in a matrix. These materials are flexible and malleable and, hence, not suitable for conversion to particulate form as pigments.
More recently, matrices of polymeric particles have been used as radiation filters. Examples of these matrices are described in a family of patents including U.S. Pat. Nos. 5,281,370; 5,711,884; 5,944,994; 6,001,251; and 6,123,845. The matrices are formed from an ordered array in a hydrogel membrane of particles of polystyrene, polymethylmethacrylate, silicon dioxide, aluminum oxide, or fluorinated polymers in a fluid medium. The array selectively filters a narrow band of wavelengths of light (radiation) from a broader spectrum of incident light. The particles are maintained in an ordered array by various techniques including evaporating the surrounding liquid and fusing the particles together, polymerizing the particles to each other, solidifying the surrounding liquid (such as by polymerization), or subjecting similarly charged particles to an electric field. The arrays are capable of Bragg diffracting radiation into reflected light and transmitted light. These gel membranes exhibit some refractive properties when broken into small pieces and mixed into a coating composition. However, their utility as a colorant in, for example, plastics or coating compositions, such as paint, is limited due to their gelatinous nature. The gelatinous materials can be readily deformed or can be swollen or de-swollen with water or organic solvents causing changes or inhomogeneities in the perceived color effect, which is undesirable.
Accordingly, a need remains for durable goniochromatic materials that can be produced in particulate form and are suitable for use as colorants.